Among traditional earphones used as acoustic devices, an earphone having a hole provided at a portion of a housing thereof and provided with an acoustically resistive member to adjust acoustic characteristics is proposed as an example for improving acoustic characteristics. A typically traditional earphone has a structure shown in FIG. 10. In FIG. 10, a bowl-shaped housing 102 accommodates a driver unit 103 therein and an opening 110 for sound emission at the front face. The driver unit 103 has a yoke 104 into which a disk-shaped magnet 105 is affixed, and a disk pole piece 106 affixed on the magnet 105. The peripheral edge of a vibrating plate 107 is supported by the inner periphery of the housing 102, and the vibrating plate 107 is configured to vibrate at a position facing the pole piece 106. The vibrating plate 107 is composed of a main dome and a surrounding subdome, and one end of a voice coil 108 is affixed along the boundary of the main dome and the surrounding subdome. The voice coil 108 is positioned in a magnetic gap between the periphery of the pole piece 106 and the inner periphery of the yoke 104.
The input of sound signals to the voice coil 108 allows the voice coil 108 to vibrate together with the vibrating plate 107 by the action of the electromagnetic force of the sound signals and the magnetic field in the magnetic gap, leading to outputting sound. A hole 109 is provided at the rear side of the housing 102, and the hole 109 is provided with an acoustically resistive member 111 on the inner face of the housing 102. The hole 109 is provided to expand the reproducible low-frequency range by relieving the pressure applied on the rear face of the vibrating plate 107 during vibration thereof, and the acoustically resistive member 111 is provided to adjust acoustic characteristics.
Furthermore, as shown in FIG. 11, a known earphone is provided with a bass compensating pipe 112 attached as a means of extending the lower-limit frequency. Its basic configuration is similar to the example shown in FIG. 10, in which the bass compensating pipe 112, having a given length and diameter, is integrated with the housing 102 having the driver unit 103 and is in communication with the internal space of the housing 102. Thus the lower-limit frequency in the vibrating plate can be expanded by the action of the space of the bass compensating pipe 112. Unfortunately, such a configuration shown in FIG. 11 restricts the shape and thickness of the driver unit 103, and thus the expansion of the internal volume of the bass compensating pipe 112, imposing limitations on improved acoustic characteristics, especially of the bass range.
Accordingly, an earphone shown in FIG. 12 is proposed. The earphone shown in FIG. 10 to FIG. 12 is disclosed in Patent Literature 1. In FIG. 12, a first housing 201 and a second housing 202 are provided, and the housings 201 and 202 communicate with each other via a communication channel 203. The first housing 201 is provided with the driver unit 103 similar to that described in FIGS. 10 and 11. A vibrating plate 204 for bass compensation is disposed in the internal space of the second housing 202. According to the description of Patent Literature 1, such a configuration achieves an earphone which has improved acoustic characteristics of the low-frequency range without any influence on those of medium to high audio frequency range.
The invention disclosed in Patent Literature 1, as shown in FIG. 12, is intended to resonate the vibrating plate 204 for bass compensation provided in the second housing 202 with the vibrating plate of the driver unit 103 so as to expand the reproducible frequency range to the lower audio frequency. Unfortunately, the drive energy from the earphone driver is essentially so small that it is impractical for the vibrating plate 204 for bass compensation to be resonated by a vibration caused by the vibrating plate of the driver unit. Even if the vibrating plate 204 for the bass compensation can be resonated, only the resonant frequency range is emphasized, so that well-balanced sound cannot be reproduced over the entire frequency range, or otherwise the driver needs to be driven at such large power as may harm an ear of a user. Furthermore, the vibrating plate 204 for the bass compensation provided inside the second housing 202 is a factor of high production costs.